Bump go to 1.13.4
Bump go to 1.13.4

* For some reason, 1.13.1 does not work well with the sync-package-specs script in diego-release

[#169683112]

cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci